Optical assembly for medical imaging devices

ABSTRACT

Optical assemblies for use in medical or other devices so as to image an object under examination onto an image sensor include a plurality of lens elements that can be retained in lens barrel. The lens elements and the lens barrel can be sealed with a compressible gasket. In one example, at least one lens element is made of an injection-moldable plastic and at least one lens element is made of a relatively dispersive optical glass. A lens barrel diameter or lens diameter can be selected to permit access to the object under examination with surgical or other tools. Aperture plates can be situated so as to reduce flare in the object image.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. provisionalapplication 60/798,492, filed May 8, 2006, that is incorporated hereinby reference.

TECHNICAL FIELD

The present invention relates to medical devices in general and, inparticular, to optical assemblies for medical imaging devices.

BACKGROUND OF THE INVENTION

As an alternative to performing more invasive procedures, manyphysicians are using minimally invasive devices to examine and/or treatinternal body tissues of patients. Examples of such minimally invasivedevices include endoscopes and catheters. Typically, such devicesinclude an elongated shaft that is inserted into the patient and amechanism for producing images from the distal end of the device. Suchmechanisms include fiber optic imaging guides that transmit light to aproximally located camera. Alternatively, the devices may include animage sensor at the distal tip that produces electronic signals that areused to produce video images of the internal body cavity. To reduce thecosts associated with repeated disinfection and repair of such devices,some endoscopes and catheters may be designed to be single-use. Examplesof single-use endoscope designs are described in U.S. patent applicationSer. Nos. 10/811,781, and 10/196,007, assigned to Boston ScientificScimed, Inc., the assignee of the present application. Theseapplications are herein incorporated by reference.

One factor that has limited the ability to make commercially feasiblesingle-use endoscopes or imaging catheters is the cost of the requiredimaging optics. To make a practical single-use device, such opticsshould provide images that are as good as or better than those obtainedwith conventional re-usable devices. In addition, these optics should beinexpensive such that the cost of the overall device allows it to beused once and thrown away. Accordingly, improved optical assemblies,endoscopes, and medical imaging devices are needed.

SUMMARY

The present disclosure pertains to optical assemblies for use in medicaldevices such as endoscopes or imaging catheters. In one example, such anoptical assembly includes a lens barrel having a number of stackedoptical elements therein. The optical elements are compressed against anO-ring within the lens barrel to form an airtight seal in the lensbarrel. In a representative example, the optical assembly includes aplurality of lens elements, and one or more of the lens elements areplastic, injection-molded lenses. These and other examples are describedbriefly in this summary so as to introduce a selection of concepts andfeatures in a simplified form that are further described below in theDetailed Description. This summary is not intended to identify keyfeatures of the claimed subject matter, nor is it intended to limit thescope of the claimed subject matter.

According to representative examples, optical assemblies include ahollow lens barrel having an inner diameter, a distal opening, and adistal rim having an inner diameter that is less than the inner diameterof the lens barrel. A compressible gasket is situated within the lensbarrel at the distal rim, and a lens is situated within the lens barreland in contact with the compressible gasket such that the compressiblegasket forms a seal between the distal rim of the lens barrel and thelens. In some examples, the compressible gasket is an O-ring. Inadditional examples, the lens is a multi-element lens and the lensbarrel includes a proximal rim that is formed over a proximal-mostoptical element of the multi-element lens so as to urge themulti-element lens to at least partially compress the compressiblegasket. In some examples, the seal formed by the compressible gasket canwithstand a pressure of at least about 30 kP.

In additional representative examples, the multi-element lens includesat least a first lens situated at the distal rim of the lens barrel andhaving a negative optical power. The first lens is in contact with thecompressible gasket so as to seal the lens barrel. In some examples, thefirst lens has a most object-wise surface that is a convex sphericalsurface and a most image-wise surface that is a concave asphericsurface. In other examples, the multi-element lens comprises, from mostobject-wise to most image-wise along a lens axis, a first lens, a secondlens, a third lens, and a fourth lens. According to illustrativeexamples, the first, second, and third lenses are made of a commonoptical plastic, and the fourth lens is made of a flint glass. In otherexamples, the first through fourth lenses have optical surfaces definedby, from a most object-wise surface to a most image-wise surface:

Surface Radius of Curvature 1 +60.000 2 +0.973 3 −8.125 4 −1.491 5+2.831 6 −1.193 7 −2.674 8 infinitewherein surfaces 2, 4, and 6 are aspheric surfaces further defined byrespective conic constants and aspheric coefficients as follows:

Surface K a₂ a₃ a₄ 2 +0.5073543 0.0 0.0 0.0 4 +0.0 −0.018796213−0.036606083 −0.068263035 6 +0.0 +0.11189657 +0.014471055 +0.044036317wherein K is a conic constant, and a₂, a₃, and a₄ are aspheric powerseries coefficients.

In representative examples, the second lens includes an object-wiseconcave optical surface and has a positive power. In additionalexamples, a first aperture plate and a second aperture plate aresituated between the first lens and the second lens, and the third lensand the fourth lens, respectively. In further examples, an aperture stopis situated between the second lens and the third lens.

Multi-element lenses configured to form an image of an object at animage surface comprise, from object-wise to image-wise along an opticalaxis, a first, a second, and a third lens element of a common opticalmaterial, and a fourth lens element of a different lens material,wherein an aperture stop is situated between the second and thirdlenses. In some examples, the first lens and the fourth lens havenegative optical power, and the second lens and the third lens havepositive optical power. In further examples, most image-wise surfaces ofthe first lens, the second lens, and the third lens are aspheric, andare defined by

K a₂ a₃ a₄ +0.5073543 0.0 0.0 0.0 +0.0 −0.018796213 −0.036606083−0.068263035 +0.0 +0.11189657 +0.014471055 +0.044036317respectively, wherein K is a conic constant, and a₂, a₃, and a₄ areaspheric power series coefficients. In some examples, an aperture platedefining a flare reducing aperture is situated between the first lensand the second lens or the third lens and the fourth lens or both. Inrepresentative examples, the first lens and the fourth lens havenegative optical power, and the second lens and the third lens havepositive optical power. In additional examples, the fourth lens includesan object-wise concave surface. In still further examples, an aperturestop is situated between the second lens and the third lens.

These and other aspects of the disclosed technology will become morereadily appreciated by reference to the following detailed descriptionand the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of a representative optical assemblyaccording to the disclosed technology.

FIGS. 2A-2B illustrate a lens barrel for use with the representativeoptical assembly of FIG. 1.

FIGS. 3A-3B illustrate an O-ring for use with the optical assembly ofFIG. 1.

FIGS. 4A-4B illustrate a first lens of the representative opticalassembly of FIG. 1.

FIGS. 5A-5B illustrate a first flare-reducing aperture plate of therepresentative optical assembly of FIG. 1.

FIGS. 6A-6B illustrate a second lens of the representative opticalassembly of FIG. 1.

FIGS. 7A-7B illustrate an aperture stop of the representative opticalassembly of FIG. 1.

FIGS. 8A-8B illustrate a spacer of the representative optical assemblyof FIG. 1.

FIGS. 9A-9B illustrate a third lens of the representative opticalassembly of FIG. 1.

FIGS. 10A-10B illustrate a second flare-reducing aperture plate of therepresentative optical assembly of FIG. 1.

FIGS. 11A-11B illustrate a fourth lens of the representative opticalassembly of FIG. 1.

FIG. 12 illustrates a representative image sensor insert configured tosituate the representative optical assembly of FIG. 1 so as to direct animage of an object to an image sensor.

FIG. 13 is a sectional view of the optical assembly of FIG. 1.

DETAILED DESCRIPTION

As used in this application and in the claims, the singular forms “a,”“an,” and “the” include the plural forms unless the context clearlydictates otherwise. Additionally, the term “includes” means “comprises.”Further, the term “coupled” means electrically, mechanically, oroptically coupled or linked and does not exclude the presence ofintermediate elements between the coupled items.

The described systems, apparatus, and methods described herein shouldnot be construed as limiting in any way. Instead, the present disclosureis directed toward all novel and non-obvious features and aspects of thevarious disclosed embodiments, alone and in various combinations andsub-combinations with one another. The disclosed systems, methods, andapparatus are not limited to any specific aspect or feature orcombinations thereof, nor do the disclosed systems, methods, andapparatus require that any one or more specific advantages be present orproblems be solved.

Although the operations of some of the disclosed methods are describedin a particular, sequential order for convenient presentation, it shouldbe understood that this manner of description encompasses rearrangement,unless a particular ordering is required by specific language set forthbelow. For example, operations described sequentially may in some casesbe rearranged or performed concurrently. Moreover, for the sake ofsimplicity, the attached figures may not show the various ways in whichthe disclosed systems, methods, and apparatus can be used in conjunctionwith other systems, methods, and apparatus. Additionally, thedescription sometimes uses terms like “produce” and “provide” todescribe the disclosed methods. These terms are high-level abstractionsof the actual operations that are performed. The actual operations thatcorrespond to these terms will vary depending on the particularimplementation and are readily discernible by one of ordinary skill inthe art.

As used herein, “lens” refers to a single optical element having tworefractive surfaces (i.e., a “singlet”) or an assembly of two or moresinglets, including optical elements that are secured to each other asin cemented doublets or otherwise fixed with respect to each other. Suchcombinations of lens elements can also be referred to as multi-elementlenses. In the disclosed examples, dioptric lenses are provided havingoptical power based entirely on refractive optical elements. In otherexamples, reflective imaging optics or catadioptric optical elements canbe provided. For convenience, light propagation from an object to animage can be illustrated with rays extending from left to right along anoptical axis that extends through one or more lens elements. Typically,an optical axis extends through a geometric center of a lens and isperpendicular to a tangent plane to the lens at the lens center. Theoptical axis can be single straight line, or one or more line segmentsif fold mirrors or other reflective optical elements such as prisms areincluded. An optical axis can be considered as extending from an object(or object surface) to an image (or image surface). Directions along theoptical axis can be referred to as object-wise (in the direction of theobject) or image-wise (in the direction of the image). These directionsare selected for convenient description as it will be appreciated thatimage and object locations can be interchanged depending on a particularapplication. For example, a lens assembly configured to produce anobject to image demagnification of ½ can be reversed to, for example,produce an object to image magnification of 2.

Surface curvatures for reflecting and refracting optical surfaces can bedescribed based on surface sag Z along an optical axis as a function ofa perpendicular distance r from a selected surface point to the opticalaxis. Using one common representation, surface sag Z(r) can be expressedas:

${{Z(r)} = {\frac{{Cr}^{2}}{1 + \sqrt{1 - {\left( {1 + K} \right)C^{2}r^{2}}}} + {a_{1}r^{2}} + {a_{2}r^{4}} + {a_{3}r^{6}} + {a_{4}r^{8}} + {a_{5}r^{10}}}},$wherein C is a surface curvature (a reciprocal of the surface radius ofcurvature), K is a conic constant K=−e², wherein e is a surfaceeccentricity that is associated with a surface corresponding to a conicsection, and a₁, a₂, a₃, a₄, and a₅ are aspheric power seriescoefficients. The value of a surface eccentricity e is greater than onefor hyperboloids, equal to one for paraboloids, between zero and one forellipsoids, and zero for spherical surfaces.

Lenses and lens elements are referred to as having a positive opticalpower if incident ray directions parallel to a lens optical axis tend tobe directed toward the optical axis after refraction. Such lens elementscan produce real images of an object. Negative optical powers areassociated with lenses that tend to direct such parallel rays away fromthe lens axis and typically do not produce real images of an objectwithout additional lens elements.

As indicated above, the disclosure pertains to optical assemblies foruse in, for example, imaging medical devices such as endoscopes orcatheters, other medical or non-medical imaging devices, and methods ofimaging for medical as well as other applications. In some convenientexamples, optical assemblies are sufficiently inexpensive to manufacturesuch that they can be incorporated into single-use medical devices. Inother examples, optical assemblies can be configured for use inre-usable devices. Improved image contrast and lens transmission can beobtained in such assemblies by providing broadband or narrowbandanti-reflective coatings on one or more lens surfaces. In onerepresentative embodiment, an anti-reflective coating is present on alllens surfaces except those of a front-most (object-wise) lens element.Light controlling apertures for flare reduction and f/number selectionare generally defined in aperture plates. Such plates can be formed of avariety of materials and are provided with suitable apertures, typicallycircular apertures configured to be centered on a lens optical axis.

As indicated above, one use of the optical assemblies disclosed hereinis in single-use medical imaging devices such as endoscopes. In oneembodiment, the optical assembly disclosed below is fitted into an imagesensor insert that supports one or more LEDs and an image sensor asdisclosed in U.S. patent application Ser. Nos. 10/811,781, and10/956,007 both of which are incorporated herein by reference. Suchendoscopes are also described in further detail below. However, otherconfigurations of the optical assembly of the disclosed opticalassemblies and an image sensor can be used.

Referring to FIG. 1, an optical assembly 5 includes a lens barrel 10 inwhich a number of lenses, aperture plates, spacers, and mounting platesas further described below are situated along an optical axis 2. Asshown in FIG. 1 and FIG. 2B, the lens barrel 10 comprises a plasticbarrel having a distal rim 14 that defines a lens barrel opening that issmaller than an inner diameter of the lens barrel 10 so as to form a lip15 that is configured to retain optical and other elements in the lensbarrel 10. A proximal rim 12 has one or more tabs or the like that canbe formed over the last (most image-wise) optical or other element ofthe optical assembly 5 by heating, bending, molding, or similartechnique in order to secure a stack of optical elements includingspacers and aperture plates in the lens barrel 10 at the proximal rim12. In a typical application, the distal rim 14 is object-wise of theproximal rim 12 so that optical radiation from an object underinvestigation is received by the optical assembly 5 at the distal rim 14and directed to an image plane or other location along the optical axis2 through the lens barrel 10. In a representative example, a maximumoutside diameter of the lens barrel 10 is about 4.75 mm and an insidediameter is about 3.6 mm, The outside diameter can be stepped so thatportions of the lens barrel have outside diameters of 4.7 mm and 4.6 mmas well.

As shown in FIG. 1, an annular O-ring or gasket 20, a first lens element30, a first aperture plate 40, a second lens element 50, an aperturestop 60, a spacer element 70, a third lens element 80, a second apertureplate 90, and a fourth lens element 100 are situated object-wise toimage-wise along the axis 2. The gasket 20 has an outer diameter thatallows it to be inserted inside the barrel 10 and an inner diameterselected so as to seal the first lens element against the lip 15 of thedistal rim 14 so as to prevent air from entering the optical assembly 5.In one example, the O-ring 20 is made of a polymer such as an ethylenepropylene co-polymer, a fluorocarbon such as vinylidenefluoride-hexafluoropropylene, acrylonitrile-butadiene copolymers, orother compliant materials.

The first lens element 30 has a negative power and is situatedimage-wise of the gasket 20. The aperture plate 40 comprises an annularring made of, for example, a black polyester material such as, forexample, polyethylene terephthalate or the like that is situatedimage-wise of the first lens 30. The aperture plate 40 defines anaperture 41 and has an outer diameter configured so that the apertureplate 40 fits within the lens barrel 10, while a diameter of theaperture 41 is selected to reduce flare in the optical assembly.

A second lens 50 having a positive refractive power is situatedimage-wise of the aperture plate 40 along the axis 2. An aperture stopplate 60 is situated image-wise of the second lens 50 and defines anaperture 61 that serves as an aperture stop for the optical assembly 5.The aperture stop plate 60 is typically an annular disk of a blackpolyester material or the like having an outer diameter selected so asto fit within the lens barrel 10. A diameter of the aperture 61 isselected to provide a desired f-number or numerical aperture for theoptical assembly 5.

A spacer element 70 is situated image-wise of the aperture stop 60 andcan be made of, for example, black plastic material or the like. A thirdlens 80 has a positive refractive power and is situated image-wise ofthe spacer 70. A second aperture plate 90 is situated image-wise of thethird lens 80 and defines an aperture 91 and can be made of, forexample, a black polyester material or the like. A fourth lens 100having a negative refractive power is situated image-wise of the secondaperture plate 90 along the axis 2.

Additional details of representative individual elements of the opticalassembly 5 are shown in FIGS. 2-11 and described further below. Unlessotherwise noted, all dimensions referred to in either this descriptionor the accompanying drawings are in millimeters. In some examples, lenselements can be conveniently manufactured by injection molding. For suchelements, both optical surface portions and mechanical mounting portionscan be provided in a common part. In the description below, portionsintended as refractive lens surfaces are referred to as optical surfaceportions.

FIGS. 2A-2B illustrate further details of the lens barrel 10. As shownin FIGS. 2A-2B, the lens barrel 10 is a tubular, hollow barrel having aproximal rim 12 and a distal rim 14. The distal rim 14 has an innerdiameter that is less than the inner diameter of the lens barrel 10 sothat the distal rim 14 forms a lip 15 to secure optical elements withinthe lens barrel 10. In one example, the proximal rim 12 includes one ormore slots or notches 16 to form tabs 22, 24 that can be heated orotherwise formed to bend over the proximal-most optical element in theoptical assembly 5, i.e., the lens 100 in the example of FIG. 1. In thisway, the lens assembly can be secured by thermal welding or with anadhesive. With the proximal rim secured over the lens 100, the lenses,spacers, aperture plates, and the aperture stop plate are retainedwithin the lens barrel 10. In one example, the lens barrel 10 is made ofan acrylic or other plastic such as, for example, C1200-HF100 plastic orother plastics, metals, ceramics, or other materials. In typicalapplications, the distal rim 14 is object-wise of the proximal rim 12along the optical axis 2.

FIGS. 3A-3B illustrate further details of the gasket 20 that isconfigured to be situated within the lens barrel 10 at the distal rim 14to form a seal between the distal rim 14 of the lens barrel 10 and thefirst lens element 30. The gasket 20 is formed of an elastomericmaterial that is somewhat compressible in order to allow the gasket 20to provide a substantially airtight seal between the distal rim 14 ofthe barrel 10 and the first lens element 30. Typically, the tabs 22, 24of the lens barrel 10 are configured to urge the lenses and otherelements of the lens assembly towards the gasket 20 so that the gasket20 presses against the distal rim 14 so as to form a seal. An aperture21 of the gasket 20 is selected to avoid hindering light transmissionthrough the optical assembly. In a typical example, an outer diameter ofthe gasket 20 is about 3.7 mm, an inner diameter is about 2.9 mm, and anuncompressed gasket thickness is about 0.3 mm. In some examples, theuncompressed gasket has a rectangular, square, or circularcross-section.

FIGS. 4A-4B illustrate a representative example of the first lenselement 30 that defines optical lens surfaces as well as convenientmounting or other mechanical features. In this example, the first lens30 has an object-side surface 32 that is substantially spherical andconvex having a radius of curvature R=+60 mm. An image side surface 33of the first lens 30 includes a concave, aspheric lens surface portion34 having a diameter of about 1.58 mm. (Surface constants for this andother surfaces are listed below in Table 2.) A center thickness of thefirst lens 30 (between the surface 32 and the lens portion 34 ) is about0.778 mm.

A V-shaped channel 35 of axial thickness of about 0.3 mm and innerdiameter of about 2.6 mm extends around the outer circumference of thelens element 30. The gasket 20 is configured to be situated at leastpartially in the channel 35 so as to seal the lens assembly. A rim 36 issituated image-wise of the channel 35 and has a diameter of about 3.6 mmand an axial thickness of about 0.3 mm and a corresponding object-wiserim 37 has a diameter of about 3.2 mm. The image side rim 36 isconfigured so serve as a spacer between the first lens 30 and anadjacent optical element. For injection-molded parts, a circumferentialportion 38 of the first lens 30 is typically flat to allow space for anygate vestige that is a by-product of the injection molding process suchthat the lens can fit into the lens barrel 10.

The first aperture plate 40 is illustrated in FIGS. 5A-5B. The firstaperture plate can be formed of a black polyester or other plasticmaterial or the like and defines an aperture 41 selected to reduceflare. Typically, the aperture plate 40 has an outside diameter of about3.55 mm and has an axial thickness of about 0.05 mm, and the aperture 41is substantially circular and has a diameter of about 1.53 mm.

A representative example of the second lens element 50 is illustrated inFIGS. 6A-6B. The second lens element 50 has an axial thickness of about0.8 mm and an outside diameter of about 3.60 mm, and includes an objectside optical surface 52 that is concave and has a radius of curvature ofabout 8.125 mm. An image side optical surface portion 54 is a convexaspheric surface situated within a tapered aperture 57 having a minimuminside diameter of about 2.2 mm. An object-wise rim 53 has an axialthickness of about 0.138 mm measured from a most object-wise surface ofthe first lens element 50 to a most-image-wise portion of the opticalsurface 52. An image-wise rim 55 has an axial thickness of about 0.54 mmmeasured between a most image-wise surface of the second lens 50 and amost image-wise portion of the optical surface 54. As shown in FIG. 6B,a circumferential portion 56 of the second lens element 50 is flattenedto allow space for any gate vestige in injection molding.

Referring to FIGS. 7A-7B, the aperture stop 60 is formed as a circulardisk about 3.55 mm in diameter and having an axial thickness of about0.05 mm, and includes a central aperture 61 having a diameter of about0.38 mm. The aperture stop 60 is conveniently formed of a black plasticmaterial such as, for example, a black polyester material such aspolyethylene terephthalate. The diameter of the central aperture can beselected based on an intended lens f-number, and in the illustratedembodiment, the central aperture diameter provides an f-number of about7.2.

As shown in FIGS. 8A-8B, the spacer 70 has an axial thickness of about0.89 mm and defines an aperture 71 having a diameter of about 2.4 mm.The spacer 70 has a flattened circumferential portion 74 for any gatevestige so that it does not protrude beyond the spacer outer diameter ofabout 3.52 mm. The spacer 70 can be formed of a black plastic such asacrylic or other plastic such as, for example, C1200 HF-100 plasticmaterial. Other plastics, metals, glasses, or ceramic materials can beused. An outer diameter of the spacer 70 includes a 45 degree chamferedportion 72.

As shown in FIGS. 9A-9B, the third lens element 80 has an object sideoptical surface portion 82 and an image side optical surface portion 84.The object side surface portion 82 is convex and spherical, and has aradius of curvature of about 2.831 mm and a diameter of about 1.7 mm.The image side surface portion 84 is a convex asphere of diameter ofabout 1.96 mm. The third lens element 80 includes a circumferentialmounting portion 81 from which the object-side optical surface portion82 extends axially object-wise about 0.131 mm and from which theimage-side optical surface portion 84 extends axially image-wise about0.14 mm. A center thickness is about 0.9 mm. The third lens element 80includes a flattened surface 86 at its circumference to facilitateinjection molding.

In representative examples, the lens elements 30, 50, 80 can be made ofZeonexB E48R plastic material from Zeon, Inc., a cyclo-olefin polymer,or other suitable lens plastics to allow the lenses to be injectionmolded. Some such optical plastics have indices of refractive n_(d) ofabout 1.5251, 1.5311, or 1.5094 at a wavelength of 587.6 nm, and Abbenumbers of about 56. The injection molded lens elements are typicallyanti-reflection coated. While plastic lenses are convenient, opticalglasses, fused silica, or other materials can be used for the lenselements 30, 50, 80. Typical materials have similar indices ofrefraction and Abbe numbers.

As shown in FIGS. 10A-10B, the second aperture plate 90 defines acircular aperture 91 having a diameter of about 1.88 mm that is selectedfor flare reduction. The aperture plate 90 is preferably made of a blackpolyester material such as, for example, polyethylene terephthalate orthe like, and is about 0.05 mm thick.

The fourth lens element 100 is illustrated in FIGS. 11A-11B. The fourthlens element is typically made of a relatively dispersive “flint” typeglass so as to provide chromatic aberration correction. In typicalexamples, ZF-52 optical glass that is available from CDGM Glass CompanyLtd is used. The lens element 100 includes an object side opticalsurface 102 having a diameter of about 2.0 mm and an image side surface105. A diameter of the fourth lens element is about 3.6 mm. The objectside optical surface 102 is concave with a radius of curvature of about2.674 mm and extends axially image-wise from a peripheral object-wiseflat portion 104 about 0.194 mm. The image side surface 105 is a flat(plano) surface. The fourth lens 100 has a center thickness of about 0.5mm.

Complete lens specifications for a representative multi-element lens arelisted in Tables 1-3. Table 1 contains basic lens data such a focallength, f-number, field of view, distortion, image field diameter, andentrance pupil diameter. Table 2 contains surface curvatures and lensmaterials. Surfaces 1, 2 are the object-wise and image-wise opticalsurfaces of the first lens, respectively, surfaces 3, 4 are theobject-wise and image-wise optical surfaces of the second lens, surfaces6, 7 are the object-wise and image-wise optical surfaces of the thirdlens, and surfaces 8, 9 are the object-wise and image-wise opticalsurfaces of the fourth lens. Surface 5 is the aperture stop. The radiusof curvature of plano surfaces is noted as infinite (“inf”) in Table 2.As discussed above, some surfaces are aspheric, and aspheric surfacesare noted with a * in Table 2. Aspheric constants for these surfaces arelisted in Table 3. Exit pupil distance is 2.02 mm nominal from the lastlens surface (i.e., the most image-wise surface of the fourth lenselement 100). A distance from the distal rim 14 of the lens barrel 10 toan image plane is 7.63 mm with a 0.5 mm glass window at an image sensor,such as a CMOS imager, with a 7 mm object distance.

In the example of Table 2, XEON E48R optical plastic is used for threelens elements (30, 50, 80) and ZF-52 glass available from CDGM GlassCompany Ltd. is used for one lens element (100). Materials from othervendors can also be used. For convenience, design refractive indices andAbbe numbers are included in Table 2.

Because this lens assembly is intended for use in forming an image of anobject on an image sensor array, a typical window for such an array isinclude in Table 1 in conjunction with surfaces 10, 11 associated with a0.5 mm thick window of CDGM Glass Company Ltd K-9 optical glass.

TABLE 1 Basic Lens Specifications Focal length 1.8 mm F# 7.2 Field ofView (FOV) 140 degrees Distortion within 15% of f-theta at maximum FOVImage Field Diameter 4.30 mm Entrance Pupil Diameter 0.22 mm ObjectDistance 10 mm Back Focal Distance 2.8 mm Total Track 7.76 mmMagnification 0.17 On-Axis Polychromatic >80% at 30 lp/mm (negativeContrast Transfer Function target) at object distance of 7 mm from firstlens vertex

TABLE 2 Lens Surface Specifications. (Surfaces noted with * areaspheric.) Radius of Refractive Abbe Surf. No. Curvature ThicknessMaterial Index Number OBJECT inf 7.000  1 (32) +60.000 0.500 E48R 1.53156.04  2* (34) +0.973 0.782  3 (52) −8.125 0.800 E48R 1.531 56.04  4*(54) −1.491 0.538  5. STOP 0.804  6 (82) +2.831 0.900 E48R 1.531 56.04 7* (84) −1.193 0.100  8 (92) −2.674 0.500 ZF52 1.847 23.8  9 (94) inf2.200 10 inf 0.500 K9 1.516 64.1 11 inf 0.138 IMAGE

TABLE 3 Aspheric Surface Specifications Surf. K a₂ a₃ a₄ 2* +0.5073543+0.0 +0.0 +0.0 4* +0.0 −0.018796213 −0.036606083 −0.068263035 7* +0.0+0.11189657 +0.014471055 +0.044036317

FIG. 13 is a sectional view of a completed lens assembly such asillustrated in FIG. 1. Some elements (70, 80) are not necessarilysymmetric about the optical axis 2 as assembled in order to, forexample, provide relief space to retain excess adhesive for assembly.The elements are typically assembled in the lens barrel in a clean roomenvironment. While the object is noted as planar in Table 2, a lensassembly can be configured for imaging of a curved object having, forexample, a radius of curvature of 10 mm, 20 mm, or other radius.

FIG. 12 illustrates one suitable environment where optical assemblies asdisclosed herein can be used. An image sensor insert 150 comprises agenerally semicircular component with a rounded upper portion 152 and agenerally flat bottom surface 154. The image sensor insert 150 isdesigned to be slidably received within a cap of an imaging endoscope orother medical device. Further details of the image sensor insert and capof the imaging endoscope are set forth in U.S. patent application Ser.No. 11/407,700, filed Apr. 20, 2006, which is herein incorporated byreference. The image sensor insert 150 includes a cooling channel 158through which a cooling liquid or gas can flow. A thermally clad circuitboard (not shown), including one or more illumination LEDs, can besituated in the cooling channel 158 in order to transfer heat from theillumination LEDs to the cooling liquid or gas. A circular bore 160 canbe provided at the center of the image sensor insert 150 or otherlocation and is configured to receive an optical assembly such as theoptical assembly 5. In a representative example, the bore 160 isprovided with a stop that aids in focusing the optical elements in theoptical assembly 5 onto an image sensor positioned on a surface 162 thatis behind and aligned with the circular bore 160. Further details ofimage sensor inserts and caps of imaging endoscopes are set forth inU.S. patent application Ser. No. 11/407,700 as noted above.

As discussed above, the optical assembly 5 can be assembled bypositioning each of the individual optical elements in the lens barrel10 in the appropriate order. The proximal-most element (the lens 100) isthen urged toward the distal end 14 of the lens barrel by the lensbarrel tabs. The various lenses, spacers, and aperture plates are thussecured in the lens barrel 10 and compress the gasket 20 so that anairtight seal is formed between the distal or object side of the opticalassembly 5 and the first lens element 30. Imaging components of anendoscope or other medical device are arranged by positioning an imagesensor behind the bore 160 in the image sensor insert 150. The opticalassembly 5 is inserted into the bore 160 and fixed with an adhesive orfastener. The illumination LEDs can be mounted on a circuit board thatis placed into the cooling channel 158.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the scope of the disclosure. For example, the lens barrelcan be formed by the bore 160 in the image sensor insert. The opticalelements can be stacked and compressed within the bore in order to sealthe assembly. While representative examples have particular focallengths, f-numbers, lens barrel outside diameters, total track lengthsand other dimensional parameters, in other examples these parameters canbe selected for a particular application. For example, a smaller outsidediameter can be provided (perhaps with a reduced focal length, increasedf-number, or a decreased lens barrel thickness or combination thereof)so as to permit a larger working channel for surgical and otherinstruments to be used based on images provided by a lens assembly/imagesensor combination. Alternatively, larger diameters, longer focallengths, reduced f-numbers, or other increased dimensional parameterscan be selected. Further embodiments of the invention may be duplicativeof or in addition to those described above. We claim all that isencompassed by the appended claims.

1. A multi-element lens, comprising a plurality of lens elementssituated so that an effective focal length is about 1.8 mm, an f-numberis less than about 8, and distortion is within about 15% off-theta atfull field of view of 140 degrees, wherein the plurality of lenselements includes, from object-wise to image-wise along an optical axis,a first lens element, a second lens element, and a third lens element ofa common optical material, and a fourth lens element of a different lensmaterial.
 2. The multi-element lens of claim 1, wherein the first,second, and third lens elements are made of a common optical plastic,and the fourth lens element is made of a flint glass, and the firstthrough fourth lens elements have optical surfaces defined by, from amost object-wise surface to a most image-wise surface: Surface Radius ofCurvature 1 +60.000 2 +0.973 3 −8.125 4 −1.491 5 +2.831 6 −1.193 7−2.674 8 inf.


3. The multi-element lens of claim 1, wherein the first, second, andthird lens elements are made of a common optical plastic, and the fourthlens element is made of a flint glass, the optical plastic has an Abbenumber of about 24 and the flint glass has an Abbe number of about 56.4. The multi-element lens of claim 1, wherein most image-wise surfacesof the first lens element, the second lens element, and the third lenselement are aspheric surfaces.
 5. The multi-element lens of claim 1,wherein most image-wise surfaces of the first lens element, the secondlens element, and the third lens element are aspheric surfaces, theaspheric surfaces of the first lens element, the second lens element,and the third lens element are defined by K a₂ a₃ a₄ +0.5073543 0.0 0.00.0 +0.0 −0.018796213 −0.036606083 −0.068263035 +0.0 +0.11189657+0.014471055 +0.044036317

wherein K is a conic constant, and a₂, a₃, and a₄ are aspheric powerseries coefficients.
 6. The multi-element lens of claim 1, furthercomprising a first aperture plate defining a flare reducing aperturesituated between the first lens element and the second lens element anda second aperture plate situated between the third lens element and thefourth lens element.
 7. The multi-element lens of claim 1, furthercomprising a lens barrel configured to receive the first, second, third,and fourth lens elements, wherein the first lens element includes acircumferential channel and a rim adjacent the channel so that acompliant gasket retained in the circumferential channel is situated tobe compressed between the rim and a lip of the lens barrel.